In conventional high speed permanent magnetic (PM) motors, magnet retention is achieved by shrinking or pressing a high strength shell over the magnets. The shell must have low permeability (preferably 1.00) to avoid short circuiting the magnet flux, have low electrical conductivity to minimize space and time harmonic losses, and comprise a thin layer of material having sufficient strength to provide for magnet retention. The thickness of the shell preferably ranges from about 0.01 inches to about 0.12 inches. Conventional shell materials are austenitic stainless steel or a heat-treatable alloy containing primarily nickel, chromium, and iron. The shell must be highly stressed to avoid separation of the magnets from the shaft at high rotor operation speeds. Thus, a tight interference fit must be used which requires large temperature differentials or force during assembly, and tight tolerances, for motors capable of operating at very high speeds. The materials that can be used for the permanent magnets are limited to those materials that can withstand the high temperatures and stresses required for fabrication.
An alternative embodiment is to turn the motor "inside-out" so that the stationary windings occupy the center position and the rotor yoke turns on the outside. In this embodiment, the magnets are held against the centrifugal forces by the yoke itself so that only some projections and adhesive are needed to position and secure the magnets. No retainer is needed in the magnetic circuit to increase the gap. Such inside-out motors typically require much more space than conventional motors and have correspondingly increased stresses thereon.
Harmonic fields resulting from inverter chopping (or pulse width modulation) and spatial variation (slotting) of stator fields often create excessive losses in magnet retainers, magnets, and rotor cores of permanent magnet motors. Analogous losses occur in induction motors from induced surface (pole face losses) and circulating currents (eddy current losses) in the rotor bars. These losses represent a substantial portion of the stray load losses present in fixed frequency induction motors.
Thin copper layers are sometimes used in conjunction with high permeability layers to exclude high frequency fields from shielded rooms. Copper has been explosively bonded to solid stainless steel slot wedges in a high speed, wound field generator to shield the slot wedges from space harmonics. These principles have not been applied to permanent magnet or induction motors.